Practice_Problems_v1 - Question 1 Anisotropic Etching a...

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Question 1 Anisotropic Etching a. Using law of cosines to calculate the angle between the <100> and the <111> crystal plane. b. In class we discussed the technique to obtain two different cavity depths using anisotropic etching by using the <111> etch rates to be finite. In this problem you will design a mask to obtain two cavities in a 500 micron wafer, one that goes through the wafer, and another that goes to half the wafer thickness. The top dimensions of the through-wafer cavity should be 500X500 microns. Assume that the <100>/<111> etch rate ratio is 300, and once the <111> planes intersect at the bottom, the resulting protruding pyramids etch instantaneously (infinitely fast). Your mask should look like one open square and one grill-like mask. c. With a certain etchant, the etch selectivity between the {100} and {111} planes is 300:1, not infinite. Therefore, the sidewalls of a pit etched in a (100) wafer will be very close to the {111} planes but not exactly parallel to them. (a) What angle do they really make? (b) What etch ratio would be needed to make 45° sidewalls? Question 2. State the main advantages and disadvantages of LPCVD compared to PECVD thin film deposition. Question 3. An SOI wafer has a 0.5 micron silicon top layer. If it is thermally oxidized completely, what will be the thickness of the resulting silicon dioxide film.
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  • Fall '07
  • Bhave
  • Orders of magnitude, Crystalline silicon, spring constant kx, etch rate ratio

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